The present invention relates to a cardiovascular valvular prosthesis and a process for manufacturing a cardiovascular prosthesis. The prosthesis advantageously solves some of the problems encountered in the cardiac valves presently known.
Persistent problems of reliability have necessitated the adoption of regulations, in many countries, for the use of said prostheses. The regulations typically contain requirements which attempt to eliminate possible disadvantages to the user and the danger arising from eventual complications subsequent to the implantation of prostheses.
Although at present different types and models of cardiac valves exist in the world, some of them, once they have been implanted in the patient, develop anomalies and problems that in certain cases may cause the death of the patient. It was evidenced in one particular case, published in the May, 1980 "Briefcase", edited by Regulatory Associates Inc., in which the manufacturer of the prosthesis, Shiley Lab. of Irvine, Calif., U.S.A., was notified by Dr. Viking Bjork, an outstanding cardio-surgeon of the Karolinska Institute of Stockholm, Sweden, of a prosthesis, 18 months after being implanted, in which the valve support had sustained a fracture.
With reference to this matter, the Executive Secretary of the Classification Panel of Cardiovascular Design in the United States has stated that the above mentioned incident is only one of many other incidents and that the majority of the valves that have failed have failed due to durability problems. Problems such as defective design and a lack of attention in the engineering details have contributed to many valve failures.
The valve, discussed above, which failed had been patented in the United States in October 1972. The valve has an occluding disc held between spaced elements which form, in a support ring, crossbars with folded portions. The longest of the crossbars have a middle portion approximately coincident with the diameter of the occluding disc, both being of a substantially "U" shape. The angle of maximum opening of the disc is 62 degrees. The disc is constructed of an acetal resin formed by the polymerization of formaldehyde having a high crystallization structure.
In the Sorin valve, the opening angle of the occluding disc is on the order of 61 to 68 degrees. The supports of the valve have a similar shape as that of the valve known as the Shiley valve discussed above.
The durability problems of the valves can be attributed to mechanical characteristics in the manufacture of the supports which are generally used in the shape in which they are cast. In other cases, the supports must be welded to the "U" shaped transversal means which support the disc.
In any of the above mentioned cases, it is almost impossible to assure that the supports will not have weak areas, either because the casting does not offer the same guarantees as the machined material or because the welded areas crack due to the many movements to which they are subjected.
It must be taken into consideration that a cardiac valve must endure an average of 45 million cycles per year when this figure is multiplied by the minimum life expectation.
Contact between valve surfaces should be limited in every way possible, in order to avoid damage to the blood through compression that could lead to irreparable clinical consequences such as microembolisms and the formation of clots through adherence. It is also desirable to obtain the greatest free circulation area possible and to offer the least resistance to the passage of the blood flow. In the case of the above mentioned valves, the support means of the occluding disc, the opening angle of which does not reach to 70 degrees, are interpolated in the flow passage which they divide, thereby creating hemodynamic resistance and the possibility of creating turbulence.
Another matter to be considered is the textile ring that is fixed to the valve. The ring is usually formed by sewing the ends to give the adequate form. This requires a transversal suture, the joining filaments of which can loosen due to failure of the ring or fatigue during its use.
In the prosthesis generally used presently, the occluding disc is not radiopaque and, thus, does not allow its visualization for control by means of X-rays during the patient's clinical followup. This prevents the effecting of a correct cardiac catheterism.
All the above mentioned disadvantages have been solved by the valvular prosthesis of the present invention, the concept of which is completely novel and the results of which have been optimum. The evaluation of prosthesis of the present invention has been effected by the Biomedical Engineering and the Mechanical Engineering Departments of Tulane University, in New Orleans, La. The evaluation was carried out in an environment which simulated the geometrical characteristics as well as the dynamics of the human left ventricle using a fluid similar to blood. The valve prototypes submitted for evaluation that corresponded to the invention were in the mitral position and in the aortic position in a chamber especially designed for the type of study conducted.
It has been found that, due to the quality of the material used for the support of the occluding disc as well as the particular shape of the disc, and as a result of its manufacturing process, the unit presents unusual physical, mechanical and chemical characteristics and superior performance. It has also been found that the support is practically inert to solutions, acids, or alkalis at room temperature.
The support is not manufactured by casting or by welding the supports of the occluding disc, as is done in the prior art. In the manufacturing process of the present invention, the morphologic structure of the alloy used is maintained without altering the position of the crystals and thus keeps the physical as well as the chemical characteristics of the support constant.
Furthermore, due to the special shape and position of the support means of the occluding disc, which does not have radial lateral arms, bulkiness at the ends, or bondings, a larger free passage area is offered without obstacles in the central portion, thereby obtaining a flow with a minimum resistance to the passage of fluid and practically eliminating the possibility of causing turbulence.
Another advantage of the present invention is that the opening of the occluding disc varies between 85 and 90 degrees, thereby providing low profile and minimum hemodynamic resistance.
Yet another advantage of the present invention lies in the shape of the occluding disc which, when in an open position, fixes a laminar flow against the faces of the disc. When the disc is in closed tangential contact, it allows a small peripheric insufficiency between the disc and the support which allows the washing of the internal surface of the support with the same blood through the action of the blood flow and the non-static accumulating of blood during the systole period when the valve is closed.
Still another advantage of the present invention is the fact that the occluding disc is manufactured with carbon substrate (graphite) with a mixture on the order of 5% of tungsten which is perfectly visible in X-rays and allows the clinical follow-up control of the patient. The material of the disc is biocompatible with blood and is thrombo-resistant.
Another advantage of the present invention is established by the construction and fixing of the textile ring or fixing hoop of the prosthesis, which does not present transversal sutures or seams for joining its ends. It is effected in one piece and thus offers higher resistance and prevents the risk of breakage by loosening or failure of the threads or fatigue during use, with the consequent risk of loosening the valve. It must be taken into account that the prosthesis, in its position, performs a slight movement on the commissural stitches as a consequence of the difference of the pressure between the systole and the diastole.
Another very important advantage of the present invention is provided by the fact that the occluding disc, due to its special assembly, can freely rotate within its support. During the operation of the prosthesis of the present invention, this free rotation allows an alternate contact of the surfaces of the disc with the support arms, thereby avoiding wear of the disc on one only surface, thereby evenly distributing the work and wear by friction or fatigue to which the disc is submitted.
In the present invention, the fixing textile ring may be freely rotated over the housing groove in the support, thereby permitting the surgeon to carry out an operation to correct in situ any variation in the position of the valve that may be considered advisable during the implantation operation and also once the valve is fixed by means of sutures. This eliminates the need to withdraw the sutures to reset the valve when the position of the valve must be corrected, thereby eliminating the consequent complications of loss of time and the eventual surgical risk for the patient.
The valve of the present invention offers excellent clinical expectations, reduces risks and the possibility of embolism and its consequences, hemodynamic turbulences and their clinical complications to the extent that, even by using non-thrombogenic materials acceptable in the manufacture of the valve, the use of anticoagulant drugs can be avoided in the post-operation treatment of the patient. At present the use of such drugs constitutes a constant preoccupation for the patient as the drugs must be injected periodically and the use extends during the patient's life time with the consequent discomfort and trauma.
Other characteristics and advantages of the present invention may be appreciated from the following description of the preferred embodiment of the present invention and the attached drawing which illustrates the cardiovascular valvular prosthesis in its preferred embodiment.